I had followed several posts about using lenses as telescopes and decided to do some experimenting. First- if you have a good spotting scope hang on to it. Using tele lenses as telescopes incorporating inexpensive parts will not produce superior images. I obtained an image quality that was acceptable to me since I only wanted something to scout a bird or animal before hiking close enough for a decent shot. That being said- I do some short distance hiking and carry a tripod, camera, 17/40, 24/105, 300 f4, and 500 f4. Even after a short walk this all gets heavy and the thought of adding a spoting scope was close to the proverbial straw that broke the camel's back. I looked for prior postings, found some, and followed the instructions using a lens cap and a 1 1/4 lens adapter. Then the fun began. I had an assortment of old eypieces from spotting scopes and experimented with them. I could get a decent image but the image was upside down and left/right reversed. My brain doesn't adapt well to that kind of viewing so I decided to try some other approaches. I tried adding a 90 degree mirror but lost focus beyond 20 ft. Added a 2X barlow-this got me right side up but still left/right reversed. The image quality was only so-so. I looked on Ebay and found a Celestron 20mm erecting image lens. It didn't work by itself but did all right when used with the barlow. This combination works only with fixed length lens because , at least with my barlow, the barlow had to go about 3/4 inch inside the lens. This means no zoom or non tele lenses. It also means no 1.4 or 2.0 extenders. I used the combo of the barlow and the erecting lens on my 300mm and 500mm with no problems BUT- proceed at your own risk and measure carefully. The total cost for the adapter, barlow and erecting lens was about $70. Perhaps if I had used better quality components the image quality would have been better but, for my purpose, image quality was adequate. Again, be very careful as you could run the risk of scratching an interior lens if you insert too far.